Lubrication of watch movement escapements that have ruby pallet-stones is a difficult process. The process consisting in arranging a drop of lubricant on the teeth of the escapement wheel does not ensure lubrication over a long period of time in that the lubricant is quickly driven towards the locking plane of the pallet stone on which it accumulates, such that it is no longer available for the desired lubrication.
CH Patent No. 116 857 proposes to overcome this problem by piercing a capillary channel through the pallet stones which extends between the locking plane and the pallet stone trailing plane, the channel acting as a tank for the lubricant. One drawback of this arrangement lies in the fact that access to the channel is awkward, so that it is difficult to fill. Moreover, given the length of the channel, it is difficult to fill it without air bubbles entering therein and preventing the lubricant from flowing freely. It is also difficult to clean such channels.
CH Patent No. 685 463 attempts to overcome these drawbacks by proposing to arrange a groove, which is open outwards and which connects the locking plane to the back plane of the pallet, on at least one of the lateral faces of the pallet stone. In addition to the fact that the lubricant in the groove is exposed to the air and thus tends to deteriorate rapidly, the shock on the pallet stone at the moment of the impulse is liable to drive said lubricant out of the groove since the capillary effect in an open groove is less than that in a closed channel.
CH Patent No. 690 013 propose to overcome the aforementioned drawbacks by making a capillary slot open onto the locking, impulse and trailing planes and in which the lubricant is received. Such an embodiment is unfavourable to the working of the escapement since the tooth of the escapement wheel moves along two faces during the impulse, which can cause extra wear.
It is also known from U.S. Pat. No. 2,685,165 a pallet-stone wherein an open capillary channel is arranged in the impulse plane thereof. The arrangement of an open channel in the impulse plane has a drawback. Indeed, in operation, when the impulse plane of the tooth of the escapement wheel transmits the impulse on the pallet via the impulse beak of the pallet-stone, a local pressure which is very high is generated at the tooth, due the pointed geometry of the impulse beak and the small contact surface of the beak with the impulse plane of the tooth. This has the effect of marking and/or deforming the impulse plane of tooth of the escapement wheel taking into account the hardness difference between the tooth of the wheel and the pallet-stone and consequently affecting the operation of the escapement. The use of an open channel causes the contact surface between the impulse beak and impulse plane of the tooth to be reduced thereby increasing the local pressure and accelerating the aging of the system.
It is an object of the present invention to overcome the aforementioned drawbacks in addition to others by providing a pallet-stone for a watch movement that ensures improved and extended lubrication of the escapement.